Gear and method for manufacturing the same

ABSTRACT

A gear that is formed of material steel molded to include a disc portion and a plurality of tooth portions circumferentially discretely formed on the disc portion and have a shape in which tooth root portions are formed between the tooth portions, and is subjected to vacuum carburizing treatment and subsequent quenching treatment with high-density energy heating after being molded.

BACKGROUND

The present disclosure relates to a gear, such as a gear for drive transmission of a vehicle, that is required to have both high hardness and high fatigue strength, and to a method for manufacturing the same. More in detail, the present disclosure relates to a gear that is made of steel as a material and has both excellent crystal grain boundary strength and excellent strength against plastic deformation, and to a method for manufacturing the same.

Examples of conventional differential gears and gears of this kind for high-load use include that described in Japanese Patent Application Publication No. 2010-001527. In Japanese Patent Application Publication No. 2010-001527, a steel type containing, for example, boron and silicon is used as material steel for a gear. The gear is subjected to vacuum carburizing at a low carbon concentration, and then quenched. Then, the entire gear is tempered. This technique is intended to obtain a gear having both high tooth root strength and high tooth flank strength.

SUMMARY

The related art described above has, however, a problem as described below. Aside from the hardness of the material steel as a bulk, the surface layer of the completed gear has insufficient fatigue strength in some cases. This problem actually causes local fatigue fracture in some cases. In particular, during the carburizing, excessive carbon penetrates to be concentrated at sharp portions, such as an edge between a tooth portion and a gear end surface, so that the quenching causes the sharp portions to have a martensitic structure having a higher carbon concentration than that in the tooth flank. As a result, the problem of insufficient fatigue strength occurs. This problem is considered to be solved by, for example, reducing the carbon concentration during the carburizing. This reduction causes the sharp portions, such as the edge, to have a martensitic structure having a lower carbon concentration than that when the carbon concentration is not reduced, so that the fatigue strength can be improved. However, this adversely reduces the carbon concentration on the tooth flank, leading to insufficient tooth flank strength. Therefore, it is not possible to satisfy both sufficiently high hardness of portions, such as the tooth flank, that need to be hard and sufficiently high fatigue strength of the sharp portions, such as the edge.

The present disclosure has been made to solve the problem involved in the related art described above. In other word, the present disclosure according to an exemplary aspect provides a gear for high-load use, such as for use in a drive transmission system of a vehicle including a differential gear, that has both sufficient hardness of tooth flanks and sufficient fatigue strength of sharp portions, such as an edge, and to provide a method for manufacturing the gear.

A drive system component according to an aspect of the present disclosure is a gear that is formed of molded material steel, includes a disc portion and a plurality of tooth portions circumferentially discretely formed on the disc portion, has a shape in which tooth root portions are formed between the tooth portions, and is subjected to vacuum carburizing treatment and subsequent quenching treatment with high-density energy heating after being molded, in which

the material steel has a chemical composition of:

C: 0.10% by mass to 0.30% by mass;

Si: 0.50% by mass to 3.00% by mass;

Mn: 0.30% by mass to 3.00% by mass;

P: 0.030% by mass or less;

S: 0.030% by mass or less;

Cu: 0.01% by mass to 1.00% by mass;

Ni: 0.01% by mass to 3.00% by mass;

Cr: 0.20% by mass to 1.00% by mass;

Mo: 0.10% by mass or less;

N: 0.05% by mass or less; and

Fe and unavoidable impurities: a residual portion, where

Si(% by mass)+Ni(% by mass)+Cu(% by mass)−Cr(% by mass)>0.5 is satisfied. A partially tempered region is provided in a surface layer of at least a part of a portion including an edge part at an end in an axial direction in the tooth portions and the tooth root portions. The partially tempered region has hardness lower than hardness of a martensitic structure generated in the surface layer of the part of the portion by the quenching treatment. A surface layer of a portion other than the partially tempered region in the tooth portions and the tooth root portions is formed by the martensitic structure generated by the quenching treatment.

The gear described above is manufactured by performing a vacuum carburizing step of heating the gear formed of the molded material steel having the chemical composition to a temperature at or higher than an austenitizing temperature of the material steel in a carburizing atmosphere at a pressure lower than the atmospheric pressure to form a carburized layer on a surface of the gear; a cooling step of cooling the gear after the vacuum carburizing step, to a temperature lower than a temperature at which structure transformation due to the cooling is completed, at a cooling rate lower than a cooling rate at which the material steel is transformed into martensite; a quenching step of heating the gear after the cooling step with high-density energy heating to increase the temperature of the gear to a temperature at or higher than the austenitizing temperature of the material steel and, from that state, cooling the gear at a cooling rate at or higher than the cooling rate at which the material steel is transformed into martensite to form a martensitic structure at least in a portion of the carburized layer; and a partial tempering step of heating at least a part of a portion including an edge part at an end in an axial direction in at least the tooth portions and the tooth root portions of the gear with the high-density energy heating after the quenching step to increase the temperature of at least the part of the portion including the edge part to a temperature of 180° C. or higher at which austenitization of the material steel does not occur and, from that state, cooling the gear to reduce a concentration of carbon dissolved in a solid state in the martensitic structure in the portion of the carburized layer in at least the part of the portion including the edge part.

In the manufacturing process of this gear, excess carbon enters the edge part during the vacuum carburizing. However, in the partial tempering step, iron carbides are formed in that part, and the concentration of the carbon dissolved in the solid state in the martensitic structure decreases. This gives the tooth flanks balanced strength at grain boundaries and in grains and improves the fatigue strength of the edge part. In this way, both the hardness of the tooth flanks and the fatigue strength of the edge part are achieved. The addition of, for example, Si secures hardenability and temper softening resistance. The partially tempered region is provided in the edge part at least at one end in the axial direction in the tooth portions and the tooth root portions of the gear to be processed. In the partial tempering step in the manufacturing process in this case, the heating should be performed with high-frequency induction heating by an exciting coil as a means of heating, in the state in which at least one end in the axial direction of the gear including the edge part lies in a space inside the exciting coil, and the other end in the axial direction of the gear lies outside the exciting coil.

The chemical composition of the material steel preferably further includes:

B: 0.005% by mass or less; and

Ti: 0.10% by mass or less.

This is because the addition of B improves the hardenability and increases the grain boundary strength of the carburized layer. The inclusion of Ti can prevent the hardenability improvement effect from being lost by B.

In more detail, examples of such a gear include, but are not limited to, a gear having a beveled shape in which an end thereof in the axial direction has a diameter larger than that of the other end. In the case of the bevel-shaped gear, the partially tempered region is provided in the edge part at the end on the larger diameter side in the tooth portions and the tooth root portions. In the partial tempering step in the manufacturing process in this case, the heating should be performed in the state in which the end on the larger diameter side of the bevel-shaped gear lies in the space inside the exciting coil, and the end on the smaller diameter side of the bevel-shaped gear lies outside the exciting coil. A bevel gear or a hypoid gear can be used as the bevel-shaped gear.

In the gear of this aspect, a meshing region on each of the tooth flanks meshing with another gear is preferably not included in the partially tempered region and is preferably formed by the martensitic structure generated by the quenching treatment. Moreover, in a side gear and a pinion gear in a differential device configured such that a plurality of gears of this aspect are meshed with each other, a meshing region on each of the tooth flanks of the gears meshing with the other of the meshing gears is preferably not included in the partially tempered region, and is preferably formed by the martensitic structure generated by the quenching treatment. This is because the meshing region needs to have high hardness.

The present disclosure provides a gear for high-load use, such as for use in a drive transmission system of a vehicle including a differential gear, that has both sufficient hardness and sufficient fatigue strength, and also provides a method for manufacturing the gear.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a differential gear according to an embodiment of the present disclosure.

FIG. 2 is a plane view showing the differential gear according to the embodiment.

FIG. 3 is a partial enlarged plan view of a part of FIG. 2.

FIG. 4 is a partial perspective view of the differential gear according to the embodiment, as viewed from another direction.

FIG. 5 is a partial sectional view of an edge part of the differential gear.

FIG. 6 is a schematic diagram illustrating an increase in fatigue strength by tempering.

FIG. 7 is a graph showing relations of quenching hardness and tempering hardness to a C concentration.

FIG. 8 is a graph illustrating an effect of tempering using a relation between surface hardness and four-point bending strength.

FIG. 9 is a front view showing a shape of a test piece and a test method used in a test shown in FIG. 8.

FIG. 10 is a sectional schematic diagram showing a heating method when partial tempering is performed.

FIG. 11 is a partial sectional view illustrating an application example to a side gear and a pinion gear in a differential device.

FIG. 12 is a graph illustrating temper softening resistance of steel of the embodiment.

FIG. 13 is a graph illustrating an influence of the C concentration in the tempering.

FIG. 14 is a graph illustrating an influence of the tempering on the hardness.

FIG. 15 is a graph illustrating an influence of tempering temperature.

FIG. 16 is an explanatory diagram showing a configuration of thermal treatment equipment suitable for thermal treatment of the embodiment.

FIG. 17 shows an example of a heat pattern of vacuum carburizing treatment and reduced-pressure slow-cooling treatment.

FIG. 18 shows an example of a heat pattern of a quenching step.

FIG. 19 shows an example of a heat pattern of a partial tempering step.

DETAILED DESCRIPTION OF EMBODIMENTS

The following describes in detail an embodiment carrying out the present disclosure, with reference to the accompanying drawings. The present embodiment carries out the present disclosure as a differential gear used in a differential device of a drive transmission system of an automobile. The perspective view of FIG. 1 and the plan view of FIG. 2 show the shape of a differential gear 1 according to the present embodiment. The differential gear 1 is discretely provided with tooth portions 11 at even intervals on the peripheral edge of a disc portion 12 having a circular shape. Tooth root portions 13 lie between the tooth portions 11. The differential gear 1 is a bevel gear having different diameters on an upper end surface 14 side and on a lower end surface 15 side in the axial direction (vertical direction in FIG. 1). The differential gear 1 shown in FIG. 1 has a smaller diameter on the upper end surface 14 side and a larger diameter on the lower end surface 15 side. FIG. 2 is a plan view of the differential gear 1, as viewed from the upper end surface 14 side having the smaller diameter. FIG. 1 shows a gear having nine teeth, and FIG. 2 shows a gear having ten teeth. While the differential gear 1 is assumed to be used as a pinion gear in a differential device, a gear used as a side gear is also the same bevel gear, except for the size and the number of teeth.

An edge part 16 of the differential gear 1 will be explained with reference to FIGS. 3 and 4. FIG. 3 is a view showing an enlarged view of a region A that is a part in FIG. 2. FIG. 4 is a partial perspective view of the differential gear 1, as viewed from a direction different from that in FIG. 1. In FIG. 4, the differential gear 1 is viewed from the lower end surface 15 side having the larger diameter. In each of FIGS. 3 and 4, hatched lines indicate a part having a projecting shape or a peak-like shape at ends on the lower end surface 15 side of the tooth portions 11 and ends on the lower end surface 15 side of the tooth root portions 13, in the differential gear 1. This part is a part greatly affected by a surface during processing of the gear. In the present disclosure, this part is called the edge part 16. The hatched region of the edge part 16 is shown in each of FIGS. 3 and 4 to particularly indicate such a part on the differential gear 1. The hatched region does not mean that some kind of matter is attached to the part on the actual differential gear 1.

FIG. 5 shows a partial sectional view of the vicinity of the edge part 16 of the differential gear 1. As well seen from FIG. 5, the vicinity of the edge part 16 has a shape projecting from another portion to form an acute angle. This shape causes the vicinity of the edge part 16 to be greatly affected by the surface during processing. FIG. 5 shows a section in the vicinity of the edge part 16 in the tooth root portion 13. The edge part 16 in the tooth portion 11 has a sharper shape than that of the portion other than the edge part 16, though not so sharp as that of the tooth root portion 13. Arrow G in FIG. 5 is shown for explanation to be given later with reference to FIG. 14.

The following describes steel that can be used as a material of the differential gear 1 (hereinafter, called “steel of the present embodiment”). Hereinafter, the unit “% by mass” in compositions will be simply noted as “%”. The components ranges of the steel of the present embodiment are as follows:

C: 0.10% to 0.30%,

Si: 0.50% to 3.00%,

Mn: 0.30% to 3.00%,

P: 0.030% or less,

S: 0.030% or less,

Cu: 0.01% to 1.00%,

Ni: 0.01% to 3.00%,

Cr: 0.20% to 1.00%,

Mo: 0.10% or less,

N: 0.05% or less, and

Fe and unavoidable impurities: a residual portion.

Furthermore, in the steel of the present embodiment, with regard to Si, Ni, Cu, and Cr among the components listed above, the following relation is satisfied:

Si(%)+Ni(%)+Cu(%)−Cr(%)>0.5.

The following describes each of the elements.

C: 0.10% to 0.30%

C is an element necessary for ensuring strength of the steel. Hence, the lower limit of the addition amount of C in the steel of the present embodiment is set to 0.1% to ensure the internal strength thereof. However, if the addition amount of C exceeds 0.30%, the following two disadvantages occur. One is that toughness decreases while hardness increases. The other is that machinability of the material steel deteriorates. Hence, the upper limit of the addition amount of C is set to 0.30%. Note that these C concentration values apply to those before a carburizing step to be described later. After the carburizing step, a surface layer portion affected thereby has a higher C concentration value than that before the carburizing step.

Si: 0.50% to 3.00%

Si is an element involved in deoxidization in a steelmaking process, and also an element effective for giving the steel necessary strength and hardenability and for improving the temper softening resistance of the steel. To obtain the temper softening resistance, the steel of the present embodiment is set to have an Si content of 0.50% or more. If the Si content exceeds 3.00%, the strength of the steel increases, so that forgeability, cold forgeability in particular, or the machinability deteriorates. Hence, the Si content needs to be in the range of 0.50% to 3.00%.

Mn: 0.30% to 3.00%

Mn is an element effective for improving the hardenability. The effect is, however, insufficient if the content of Mn is less than 0.30%. The Mn content of more than 3.00% causes, however, an increase in the hardness, so that the forgeability, cold forgeability in particular, or the machinability deteriorates. Hence, the Mn content needs to be in the range of 0.30% to 3.00%.

P: 0.030% or less

P has an effect of reducing the toughness by grain boundary segregation. Therefore, the content of P needs to be reduced to minimum. The P content can hardly be reduced to zero, but needs to be limited to 0.030% or less.

S: 0.030% or less

S has an effect of reducing the ductility by reacting with Mn in the steel to generate MnS. Therefore, the content of S needs to be 0.030% or less.

Cu: 0.01% to 1.00% and

Ni: 0.01% to 3.00%

In addition to Si described above, Cu and Ni are components that inhibit generation of iron carbides. Hence, the steel of the present embodiment is set to have Cu and Ni contents of 0.01% or more each. However, excessive contents of Cu and Ni deteriorates hot processability, so that the Cu content needs to be 1.00% or less, and the Ni content needs to be 3.00% or less.

Cr: 0.20% to 1.00%

In contrast to Si, Cu, and Ni, Cr is a component that facilitates the generation of the iron carbides, and should not be contained in the steel in a large amount. Hence, the content of Cr needs to be kept at 1.00% or less. The same also applies in the case in which the steel contains a relatively large amount of components inhibiting the generation of the iron carbides. On the other hand, Cr is an element that improves the hardenability and the temper softening resistance of the steel, so that the Cr content needs to be 0.20% or more.

As described above, Si, Cu, and Ni have the effect opposite to that of Cr, regarding the generation of the iron carbides. In the steel of the present embodiment, the generation inhibiting effect of Si, Cu, and Ni needs to exceed the generation facilitating effect of Cr. Hence, the sum of the contents of Si, Ni, and Cu needs to exceed the content of Cr by a difference of 0.50 or more.

Mo: 0.10% or less

Mo is not an essential element in the steel of the present embodiment, and must not exceed the upper limit content of 0.10%, if contained. Provided that the upper limit given above is not exceeded, the hardenability and the temper softening resistance can be expected to be improved by containing Mo. The same effect is, however, obtained by adding an appropriate amount of Si or Mn (Si, in particular), so that the inclusion of Mo is not essential.

N: 0.05% or less

An excessive amount of N in the steel significantly deteriorates the forgeability. Here, for example, Ti fixes N, thereby reducing the influence of N on the forgeability. In this case, N reacts with Ti in the steel to generate a nitride. However, generation of large-size TiN particles causes a reduction in strength. Therefore, the content of N needs to be 0.05% or less.

The steel of the present embodiment may further contain the following components:

B: 0.005% or less (not including 0%) and

Ti: 0.10% or less (not including 0%).

B: 0.005% or less (not including 0%)

Adding B gives the steel the hardenability. Thus, B is an element effective for increasing grain boundary strength. B increases the grain boundary strength because B takes priority over P to segregate at grain boundaries in the steel. Although it is a known fact that the grain boundary segregation of P markedly reduces the grain boundary strength of the steel, B prevents such reduction in the grain boundary strength. The grain boundary segregation of B has, in fact, an effect of improving the grain boundary strength of the steel. However, an excessive content of B saturates the advantageous effect on the hardenability, and also impairs processability. Therefore, the content of B needs to be 0.005% or less.

In particular, the addition of B has a great significance when a steel product to be processed is subjected to phosphating treatment in a molding process. This is because P contained in the phosphate film enters the steel to a certain extent during the carburizing treatment. Drive system components, such as the differential gear 1, are often subjected to the phosphating treatment in the molding process, and are improved in the grain boundary strength by the addition of B.

Ti: 0.10% or less (not including 0%)

Ti is an element that reacts with N in the steel to generate the nitride (TiN). Hence, Ti has an effect of preventing B from reacting with N to change into BN, and thereby having an effect that prevents loss of the effect of B for improving the hardenability. Ti also has an effect of reducing deformation resistance of the steel by reacting with N to reduce the solid solution amount of N in the crystal lattice of iron. Generation of large-size TiN particles, however, causes a reduction in the strength of the steel. Therefore, the content of Ti needs to be 0.10% or less.

In the following description, the material steel used for the differential gear 1 or test pieces thereof has the following component composition, if not otherwise specified:

C: 0.18%,

Si: 0.75%,

Mn: 0.40%,

P: 0.015%,

S: 0.015%,

Cu: 0.15%,

Ni: 0.10%,

Cr: 0.35%,

Mo: 0.07%,

B: 0.002%,

Ti: 0.040%,

Fe and unavoidable impurities: a residual portion.

The following describes thermal treatment applied to the differential gear 1. The steel having the components listed above as a starting material is roughly molded by cold forging, and then is cut to be formed into the differential gear 1 of the present embodiment. In this way, the outer shape of the differential gear 1 is completed. The differential gear 1 of the present embodiment is, however, obtained by being further subjected to treatment of the following steps.

1. Carburizing Step

This is a step of applying treatment of forming a carburized layer in the surface layer portion of the differential gear 1 by heating the differential gear 1 in a carburizing atmosphere so as to increase the hardness of the surface layer portion.

2. Cooling Step

This is a step of cooling the differential gear 1 after the carburizing step. This cooling needs to be performed at least until structure transformation due to temperature drop is completed after the carburizing step.

3. Quenching Step

This is a step of heating the differential gear 1 after the cooling step with high-density energy to an austenite region, and rapidly cooling the differential gear 1 after heating so as to harden the differential gear 1.

4. Tempering Step

This is a step of locally applying tempering to a region where a large amount of carbon has entered at the carburizing step as a step 1.

The carburizing step as the step 1 will further be described. In general, in the carburizing step, a hydrocarbon-based gas is introduced into a furnace, and a steel material to be processed (here, material of the differential gear 1) is placed in the atmosphere of the gas and heated to an austenitizing temperature or higher. Thus, C enters the surface layer of the steel material to be processed to form the carburized layer. In this carburizing treatment, first, in the carburizing period, molecules of the carburizing gas come in contact with the surface of the steel and are decomposed to generate activated carbon (C). The activated C is supplied to the surface of the steel so as to form the carbides. This process stores C on the surface of the steel. During the subsequent diffusion period, the carbides are decomposed, so that the stored C dissolves in the matrix of Fe. This process diffuses the carbon inward to form the carburized layer. The entering route of the carbon is not limited to the route via the carbides, but there is also a route along which the activated C directly dissolves in the matrix.

The carburizing step in the present embodiment is performed using vacuum carburizing treatment, in which the temperature is set in the range of 900° C. to 1100° C., and the ambient pressure is set lower than the atmospheric pressure. This process regulates the C concentration in the surface of the steel material after the diffusion period to 0.8% or less, which is relatively low for a C concentration after the carburizing. In this way, the C concentration in the carburized layer is regulated to equal to or below the amount of carbon of eutectoid steel. As a result, after the steel material is heated during the subsequent quenching step to be transformed into austenite again and is then rapidly cooled, a martensitic structure can be formed without precipitation of the iron carbides (such as cementite). The term “martensitic structure” used herein may include 20% or less of a retained austenite. If the carburizing step increases the C concentration in the surface to a level exceeding 0.8%, the iron carbides (such as cementite) segregate at grain boundaries after the quenching. The grain boundaries where the iron carbides segregate can serve as a starting point of breakage and reduce cyclic strength. In the case of the differential gear 1 of the present embodiment, such a phenomenon is prevented by keeping the C concentration after the carburizing step at a relatively low value. In the carburizing step described above, the carburizing temperature is preferably about 1000° C.

In the vacuum carburizing step described above, the ambient pressure is preferably in the range of 1 hPa to 20 hPa. Reducing the ambient pressure in the vacuum carburizing step to lower than 1 hPa necessitates high-cost equipment in order to achieve and maintain the vacuum degree. On the other hand, increasing the ambient pressure to a high pressure exceeding 20 hPa generates soot during the carburizing. This can cause a problem of uneven carburizing. As the carburizing gas described above, the hydrocarbon-based gas, such as acetylene, propane, butane, methane, ethylene, or ethane, can be used.

In the differential gear 1 of the present embodiment, the amount of carbon that has entered during the carburizing treatment varies depending on the location due to the shape of the differential gear 1. That is, at the edge part 16 described with reference to FIGS. 3 to 5, the amount of carbon is larger than that in the other portion. This is because the vicinity of the edge part 16 has a sharper shape, so that the carbon that has entered from the surface can diffuse inward into a portion having only a small volume, and, as a result, a large amount of C is present after the carburizing. This is also because the vacuum carburizing is performed in the carburizing step. With gas carburizing in which the ambient pressure is set to the atmospheric pressure, the surface of the steel material is brought into an equilibrium state, in which a decarburizing reaction occurs, in addition to the carburizing reaction. As a result, the C concentration in the edge part 16 is not necessarily higher than that in the other portion. In the vacuum carburizing, however, the reaction progresses in a nonequilibrium state, in which only the carburizing reaction occurs, and the decarburizing reaction does not occur. As a result, the concentration of C occurs in the edge part 16.

The following describes the cooling step as a step 2. The cooling step is performed under a slow-cooling condition. More specifically, at least, the steel material of the differential gear 1 is cooled to below a temperature at which structure transformation due to the cooling is completed, at a cooling rate lower than a cooling rate at which the steel material of the differential gear 1 is transformed into martensite during the cooling. This cooling method can suppress the occurrence of distortion associated with the martensitic transformation. As a result, the carburizing treatment can be completed with excellent shape accuracy.

Such an effect of the cooling step can suppress the distortion during the cooling after the carburizing, whereby the process proceeds to the next step, that is, the quenching step, while maintaining the high dimensional accuracy. This effect is obtained at a higher degree by performing the cooling step under the slow-cooling condition. When this effect is combined with an advantage obtained by performing the subsequent quenching step with high-density energy heating, the shape of the differential gear 1 after the quenching can be highly accurate with low distortion.

In addition, the cooling step is preferably performed under reduced pressure, as in the carburizing step. In this case, the pressure difference is small between the two steps. This allows, in the actual equipment, the two steps to be continuously performed by directly connecting between a carburizing chamber and a slow-cooling chamber. That is, for example, a preliminary chamber for pressure adjustment need not be provided between the two chambers. In other words, the product after being subjected to the vacuum carburizing treatment can be subjected to the reduced-pressure slow-cooling treatment without being exposed to the atmospheric pressure. This also contributes to the reduction in the distortion. In this case, the ambient pressure at the cooling step is preferably in the range of 100 hPa to 650 hPa. The cooling step can be performed under non-reduced pressure.

Next, the quenching step as a step 3 will be described. In the quenching step, it is important to heat the differential gear 1 to the austenitizing temperature or above, and, from that state, to rapidly cool the differential gear 1 so as to transform at least the portion of the carburized layer into martensite. For this purpose, the differential gear 1 once cooled at the cooling step as the step 2 is increased in temperature again to a high temperature. High-energy heating, such as high-frequency induction heating, is suitable for this heating.

The rapid cooling in the quenching step is preferably performed by water cooling. That is, the rapid cooling by the water cooling can cause the martensitic transformation, so that a high quenching effect is obtained. In other words, higher strength of a quenched portion is achieved. When the differential gear 1 is heated by the high-frequency induction heating, the differential gear 1 is preferably processed one by one. When the differential gear 1 is water-cooled after being heated, the differential gear 1 is preferably cooled by being rotated and sprayed with cooling water from around. In this way, various portions of the differential gear 1 can be uniformly cooled. As a result, occurrence of the distortion due to the rapid cooling is suppressed. In addition, as described above, the martensitic structure without precipitation of the iron carbides is obtained in the quenched portion of the differential gear 1.

In the present embodiment, the differential gear 1 is an object to be processed. When the object to be processed includes the projecting tooth portions 11 as in the case of the differential gear 1, the heating in the quenching step is preferably performed under a condition in which all of the surface and the inside of the tooth portions 11 are austenitized. This is because the differential gear 1 needs to have both high surface hardness of the tooth portions 11 and high ductility of the inside thereof. Therefore, the high-density energy heating is suitable as a method for heating in the quenching step.

The following describes the tempering step as a step 4. This tempering step aims not at tempering the entire differential gear 1, but at locally tempering a particular region. The particular region to be tempered is the edge part 16 described with reference to FIGS. 3 to 5. The reason for this is as follows: while having the high surface hardness, the differential gear 1 after the quenching step has low fatigue strength at the edge part 16, and the local tempering aims at solving this problem.

As described above, the edge part 16 is a place where a large amount of C enters compared with the other portion during the carburizing step. As a result, the concentration of C dissolved in the solid state in the martensitic structure after the quenching is also high compared with the other portion. This causes the martensitic structure in old austenite grains to have hardness higher than normal. However, this result adversely reduces the fatigue strength. This is because the martensitic structure in the old austenite grains has too high hardness, so that a load generated when a stress is applied concentrates only on grain boundaries. As a result, as shown in FIG. 6, a crack 3 is generated at grain boundaries 4, so that a gap is produced between crystal grains 2. This results in breakage at the edge part 16 during an endurance operation.

To solve this problem, a portion in the vicinity of the edge part 16 (at least a part of the differential gear 1 including the edge part 16) in the differential gear 1 of the present embodiment is locally tempered. Specifically, the temperature of the vicinity of the edge part 16 of the differential gear 1 is increased to a value in the range of 180° C. to 500° C. at which the austenitization does not occur, followed by cooling. The method of cooling may be water-cooling or air-cooling, but the water-cooling is better because the cooling rate should be higher. With this method, the concentration of C dissolved in the solid state in the martensitic structure in the old austenite grains becomes lower than that before the tempering, while the carbon concentration in the steel does not decrease in the region near the edge part 16. As a result, the hardness of the martensitic structure in the old austenite also becomes lower than that before the tempering. This results in the load generated when a stress is applied uniformly acting on the grain boundaries and inside the grains. In this way, the crack 3 shown in FIG. 6 is prevented from occurring. In other words, the strength during the endurance operation increases.

The following describes the decrease in the hardness by the tempering, with reference to the graph in FIG. 7. The graph indicates the surface hardness (HV) of the steel material of the differential gear 1 before and after the tempering against each surface layer C concentration. In the graph, the plot labeled “QUENCHING” represents the hardness before the tempering, and the plot labeled “180° C. TEMPERING” represents the hardness after the tempering. By comparing, in this graph, the hardness for the same C concentration between before and after the tempering, it is found that the hardness after the tempering is lower than that before the tempering. For example, when the C concentration is 0.6% in the graph, the hardness is approximately HV 770 before the tempering, and drops to approximately HV 700 after the tempering. This indicates the hardness decreasing effect caused by the tempering. In this way, the tempering slightly reduces the hardness of the locally tempered portion near the edge part 16.

The tempering is considered to reduce the hardness in the following way. That is, the tempering causes a part of C dissolved in the solid state in the martensitic structure in the old austenite grains to form the carbides together with Fe. The concentration of C dissolved in the solid state in the martensitic structure in the old austenite grains decreases by an amount corresponding to the formation of the carbides, so that the hardness decreases. That is, the hardness is reduced to lower than the hardness of the martensitic structure before the tempering at the same C concentration. The hardness before the tempering is maintained in places other than the locally tempered place even on the surface layer of the differential gear 1. This is because no change occurs in the concentration of C dissolved in the solid state in the martensitic structure in the old austenite grains.

Consequently, in the locally tempered portion near the edge part 16, the carbides of Fe are generated in an amount corresponding to the decrease in the concentration of C dissolved in the solid state in the martensitic structure in the old austenite grains. As a result, the existence ratio of the carbides of Fe in this portion is larger than that in the other portion. This can be confirmed by comparing the ratio of the area on the surface occupied by the carbides of Fe between this portion and the other portion. The carbides of Fe mainly consist of ε-carbide (Fe₂₋₃C) and cementite (Fe₃C), and the generation ratio therebetween varies depending on the increased temperature during the tempering. More ε-carbide is generated than cementite when the increased temperature during the tempering is in the range of 180° C. to 250° C. More cementite is generated than ε-carbide when the increased temperature during the tempering is in the range of 250° C. to 500° C.

The following describes the effect of the tempering, with reference to the graph in FIG. 8. This graph is a graph showing a relation between the surface hardness (HV) and 10000-cycle strength (MPa) regarding the steel of the present embodiment. The surface hardness refers to Vickers hardness. The 10000-cycle strength refers to the maximum stress that can be withstood after being repeatedly applied 10000 times. This repeated test was conducted using a round bar-like test piece 20 having a notch 21, as shown in FIG. 9. Vickers hardness was measured at the bottom of the notch 21 of the test piece 20 before the repeated test. The surface hardness was changed by carburizing and tempering the test piece 20.

FIG. 8 shows that the surface hardness has a downward-sloping relation, that is, an inverse relation with the 10000-cycle strength. The plotted points of Group D in FIG. 8 represent results for the test piece 20 having a relatively lower concentration of C dissolved in the solid state in the martensitic structure, and indicate that the test piece is exceptionally superior in the 10000-cycle strength while being slightly inferior in surface hardness. This group corresponds to the portion other than the edge part 16 and the edge part 16 after the tempering in the differential gear 1. The plotted points of Group E represent results for the test piece 20 having a relatively higher concentration of C dissolved in the solid state in the martensitic structure, and indicate that the test piece is exceptionally superior in surface hardness while being slightly inferior in the 10000-cycle strength. This group corresponds to the edge part 16 before the tempering in the differential gear 1. The above shows that the tempering slightly reduces the surface hardness from the hardness before the tempering, but is effective for improving the 10000-cycle strength (that is, the fatigue strength).

The local heating for the tempering is performed in the following way. FIG. 10 schematically shows an arrangement relation between a heater and the differential gear 1 in the case of performing the heating with the high-frequency induction heating. FIG. 10 shows a circular ring-like exciting coil 22 and a bar-like sample holder 23 as constituent components of a high-frequency induction heater. The high-frequency induction heater uses the sample holder 23 to clamp the differential gear 1 shown in FIGS. 1 to 5 from above and below so as to support the differential gear 1, and moves the differential gear 1 in the axial direction, that is, in the vertical direction in FIG. 10 so as to arrange the differential gear 1 in a space inside the exciting coil 22. In that state, the high-frequency induction heater applies a high-frequency current to the exciting coil 22 to heat the differential gear 1 using an electromagnetic induction effect caused by the high-frequency current.

During the local heating of the present embodiment, the lower end surface 15 of the differential gear 1 having the larger diameter faces the exciting coil 22, as shown in FIG. 10. The arrangement relation is such that instead of the entire differential gear 1, only the vicinity of the lower end surface 15 lies in the space inside the exciting coil 22, and the smaller-diameter portion on the upper end surface 14 side lies outside the exciting coil 22. The coil is excited in this state, so that the edge part 16 and the vicinity thereof on the lower end surface 15 side are locally heated, but the portion on the upper end surface 14 side is less heated. The differential gear 1 is then cooled to be partially tempered.

Partially tempering the differential gear 1 does not mean that the portion other than the edge part 16 is completely unaffected by the tempering. However, the steel of the present embodiment secures the Si content of 0.50% or more as described above. This gives the differential gear 1 high temper softening resistance. Accordingly, even the low C-concentration region other than the edge part 16 has sufficient hardness after the tempering.

The following describes a case in which gears each corresponding to the “differential gear 1” described above are used as a side gear and a pinion gear in the differential device, with reference to FIG. 11. FIG. 11 is a partial sectional view showing a meshing portion between a side gear 100 and a pinion gear 200 in the differential device. Each of the side gear 100 and the pinion gear 200 in FIG. 11 is a gear corresponding to the “differential gear 1”.

The side gear 100 in FIG. 11 is arranged so that its axial direction corresponds to the horizontal direction in FIG. 11. The left side in FIG. 11 corresponds to a larger diameter surface 115. The pinion gear 200 is arranged so that the axial direction thereof corresponds to the vertical direction in FIG. 11. The upper side in FIG. 11 corresponds to a larger diameter surface 215. Regions where a tooth portion 111 of the side gear 100 overlaps a tooth portion 211 of the pinion gear 200 in FIG. 11 correspond to meshing regions 117 and 217 between tooth flanks of the two gears.

FIG. 11 indicates edge parts 116 and 216 of the side gear 100 and the pinion gear 200, respectively, with surrounding dashed lines. Both these parts have the excellent 10000-cycle strength obtained by the tempering as described above. On the other hand, both the meshing regions 117 and 217 are found to belong to the portion other than the partially tempered region. Therefore, both the meshing regions 117 and 217 have sufficiently high hardness as described above.

This will be described with reference to the graph in FIG. 12. The graph in FIG. 12 is a graph showing a relation between the surface layer C concentration and the Vickers hardness after the tempering for each of a low Si material (with an Si concentration of 0.18%) and the steel of the present embodiment (with an Si concentration of 0.75%). The low Si material shown in FIG. 12 has a surface layer C concentration in the range of 0.5% to 1.1%. This range was achieved by the gas carburizing. This low Si material shows the highest post-tempering hardness when the surface layer C concentration is 0.8%.

The steel of the present embodiment represented in the graph was obtained by setting the C concentration after the carburizing to a relatively low value as described above on the assumption that the C concentration represents that of the portion other than the edge part 16 of the differential gear 1. Although having a low surface layer C concentration of 0.6%, the steel of the present embodiment represented in this graph has hardness equal to the highest hardness of the low Si material obtained when the surface layer C concentration is 0.8%. This is the effect of the temper softening resistance achieved by adding Si.

The graph of FIG. 13 is a graph for explaining an influence of the C concentration on the fatigue strength. This graph shows the number of cycles of repeated application of a constant stress required to cause fracture. In the graph, the plot labeled “EXCESS C %” was obtained using a test piece with a surface layer C concentration increased to 0.8% or more by the gas carburizing, and represents a comparative example. The plot labeled “HIGH C %” was obtained using a test piece with a surface layer C concentration after the carburizing set in the range of 0.6% to 0.8%, and corresponds to the edge part 16 of the differential gear 1. The plot labeled “LOW C %” was obtained using a test piece with a surface layer C concentration after the carburizing set in the range of 0.3% to 0.6%, and corresponds to the portion other than the edge part 16 of the differential gear 1.

According to any of the plots “EXCESS C %”, “HIGH C %”, and “LOW C %” in FIG. 13, the number of cycles (horizontal axis) decreases as the applied stress (vertical axis) increases. According to a comparison at Arrow F (where the number of cycles is 3000), the plot “HIGH C %” represents a stress value higher by approximately 15% than that of the plot “EXCESS C %”. The plot “LOW C %” represents a stress value higher by approximately 40% than that of the plot “EXCESS C %”. This is the effect of the fatigue strength improvement achieved by reducing the C concentration. These tests were conducted by performing four-point bending of the round bar-like test piece 20 shown in FIG. 9.

Next, an influence of the tempering on the hardness will be described with reference to FIG. 14. FIG. 14 shows relations between the Vickers hardness and the depth from a surface H of the differential gear 1 on Arrow G in the sectional view shown in FIG. 5. Before the tempering, a region in the surface layer at a depth of 1 mm or less shows markedly higher Vickers hardness than that in a core portion at a depth of 1 mm or more. This is considered to be because of the concentration of C in the edge part 16 during the carburizing described above. The hardness after the tempering is slightly lower than that before the tempering, but is not lower than the hardness of the core portion before the tempering. These results show that sufficient hardness is maintained after the tempering. The results of the tempering in the test in FIG. 14 are those obtained in the case in which the high-frequency induction heater was used to heat the test pieces under conditions of 4.5 kHz, 110 V, and 4 seconds. Under these conditions, the temperature of the surface of the edge part 16 reached approximately 190° C.

The following describes an influence of the tempering temperature with reference to FIG. 15. FIG. 15 is a graph showing a relation, for each tempering temperature, between the surface layer C concentration (%) and 6400-cycle strength (MPa) in the four-point bending test of the round bar-like test piece 20 shown in FIG. 9. In this graph, the surface layer C concentration (%) represents the C concentration at the bottom of the notch 21 of the test piece 20 after the carburizing. The 6400-cycle strength refers to the maximum stress that can be withstood after being repeatedly applied 6400 times.

FIG. 15 shows the results at various C concentrations for three levels of “no tempering”, “180° C. tempering”, and “400° C. tempering”. At each of the C concentrations, the results of both the 180° C. tempering and the 400° C. tempering show higher 6400-cycle strength than that of the no tempering. According to a comparison of the values of these results at a C concentration of 0.56% with the value labeled “CONVENTIONAL PRODUCT” in FIG. 15, the 180° C. tempering increases the 6400-cycle strength by approximately 20%, and the 400° C. tempering increases the 6400-cycle strength by approximately 23%. An example of 500° C. tempering at a C concentration of 0.56% is also plotted in FIG. 15. This example achieves an increase of approximately 28% from the “CONVENTIONAL PRODUCT”. The above shows that the tempering temperature is preferably higher in the range of 180° C. to 500° C. for improving the fatigue strength.

However, in the case of tempering the differential gear 1 to improve the fatigue strength of the edge part, it is not preferable to temper the differential gear 1 at a higher temperature (for example, in the range of 300° C. to 500° C.) because the heat for the tempering reaches the tooth flanks of the differential gear 1 to reduce the hardness of the tooth flanks. It is also not preferable to set the tempering temperature in the range of 200° C. to 300° C., which is called a tempering brittleness range, because the steel becomes more brittle than that before the tempering. For the reasons described above, the tempering temperature of the differential gear 1 is preferably in the range from 180° C. to below 200° C.

The following briefly describes thermal treatment equipment suitable for carrying out the steps from the carburizing step to the tempering step described above. As shown in FIG. 16, this thermal treatment equipment 5 suitable for the present embodiment includes a pre-cleaning bath 51, a vacuum carburizing slow cooling device 52, an induction hardening machine 53, an induction tempering machine 54, and a magnetic flaw detecting device 55. The pre-cleaning bath 51 is a section for cleaning the differential gear 1 before starting the thermal treatment. The vacuum carburizing slow cooling device 52 includes a heating chamber 521, a vacuum carburizing chamber 522, and a reduced-pressure slow cooling chamber 523. The differential gear 1 is increased in temperature in the heating chamber 521, and is subsequently subjected to the vacuum carburizing (the step 1) performed in the vacuum carburizing chamber 522 and the reduced-pressure slow-cooling (the step 2) performed in the reduced-pressure slow cooling chamber 523. No preliminary chamber is provided between the vacuum carburizing chamber 522 and the reduced-pressure slow cooling chamber 523. The induction hardening machine 53 is a section for applying the high-frequency induction heating and the subsequent water cooling (the step 3) to the differential gear 1 after the reduced-pressure slow cooling. The induction tempering machine 54 is a section for applying the partial tempering (the step 4) to the differential gear 1 after the quenching, by applying thereto the high-frequency induction heating and the subsequent water cooling. The magnetic flaw detecting device 55 is a section for inspecting defects of the differential gear 1 after the tempering.

Subsequently, each step performed by the thermal treatment equipment 5 in FIG. 16 will be described. First, the vacuum carburizing step (the step 1) performed in the vacuum carburizing chamber 522 of the vacuum carburizing slow cooling device 52 will be described. The carburizing treatment in the present embodiment is the vacuum carburizing treatment performed in the carburizing gas that is reduced in pressure to a pressure lower than the atmospheric pressure. FIG. 17 shows a heat pattern in the vacuum carburizing treatment and the subsequent reduced-pressure slow-cooling treatment. In FIG. 17, the horizontal axis represents time, and the vertical axis represents temperature.

In FIG. 17, the symbol “a” represents a heating period in the heating chamber 521. The symbols “b1” and “b2” represent a holding period in the vacuum carburizing chamber 522. The earlier period “b1” of the holding periods corresponds to the carburizing period in the carburizing treatment, and the subsequent later period “b2” corresponds to the diffusion period in the carburizing treatment. For the differential gear 1 and the test piece 20 used in the tests described above, the carburizing temperature, that is, a holding temperature in the holding periods “b1” and “b2” was set to 950° C. that is a temperature at or above the austenitizing temperature of the material steel. That is, the temperature of the differential gear 1 was increased to the holding temperature in the heating period “a”. The temperature of the differential gear 1 was maintained at a constant temperature, that is, at the holding temperature described above, during the holding periods “b1” and “b2”.

For the differential gear 1 and the test piece 20 used in the tests described above, the pressure of the carburizing gas in the vacuum carburizing treatment was set in the range of 1 hPa to 3.5 hPa. Acetylene was used as the carburizing gas in the carburizing period “b1 “. Through experiments for condition setting conducted in advance, carburizing conditions were set as follows: that is, conditions were employed under which the C concentration in the surface layer of the edge part 16 is in the range of 0.6%±0.05%, and the C concentration in the surface layer of a portion (such as the tooth flank) away from the edge part 16 is in the range of 0.5%±0.05%.

Subsequently, the following describes the reduced-pressure slow-cooling step (the step 2) performed in the reduced-pressure slow cooling chamber 523 subsequent to the vacuum carburizing treatment. As described above, the slow-cooling treatment in the present embodiment is the reduced-pressure slow cooling treatment that is performed in an atmosphere at a pressure reduced to lower than the atmospheric pressure. In FIG. 17, a period represented by the symbol “c” is a slow cooling period. For the differential gear 1 and the test piece 20 used in the tests described above, the ambient pressure in the reduced-pressure slow cooling treatment was set to 600 hPa. The type of the atmosphere gas was N₂ gas. The cooling rate in the reduced-pressure slow cooling treatment was set in the range of 0.1° C./s to 3.0° C./s. The cooling was performed at this cooling rate until the temperature decreases from a temperature at or above the austenitizing temperature immediately after the carburizing treatment to 150° C. that is lower than the A1 transformation point. The heat pattern shown in FIG. 17 is an example, and can be changed, by conducting appropriate preliminary tests, to a heat pattern giving optimal conditions for the type of material steel used.

Subsequently, the quenching step (the step 3) in the induction hardening machine 53 will be described. In the quenching step for the differential gear 1 and the test piece 20 used in the tests described above, high-frequency induction heating was used as a means of high-density energy heating. The water cooling was used as a means of rapid cooling. The heat pattern shown in FIG. 18 was employed as the heat pattern of the quenching step. In FIG. 18, as in FIG. 17, the horizontal axis represents time, and the vertical axis represents temperature. In FIG. 18, the symbol “d1” represents a temperature increasing period, and the symbol “d2” represents a rapid cooling period. During the temperature increasing period “d1 ”, the high-frequency induction heating is used to heat the tooth portions 11 on the outer circumferential side of the differential gear 1 to a temperature at or above the austenitizing temperature. During the subsequent rapid cooling period “d2”, sprayed water rapidly cools the differential gear 1 so that the cooling rate of the carburized layer thereof reaches the critical cooling rate or higher. The critical cooling rate refers to a cooling rate required for transforming the austenitized material steel, particularly the carburized layer portion thereof, into martensite, as described above.

For the differential gear 1 and the test piece 20 used in the tests described above, the high-frequency induction heating during the temperature increasing period “d1” was conducted as follows: the amount of input energy was set smaller than an amount given under conditions for ordinary high-frequency induction heating, and the heating time was set in the range of 15 seconds to 25 seconds, which is relatively longer by an amount corresponding to the difference in the amount of input energy. In this way, the entire tooth portions 11 including the vicinity of the surfaces and the inside thereof was heated to a temperature in the range of 900° C. to 1000° C. The temperature on the surfaces of the tooth root portions 13 reached a value in the range of 920° C. to 940° C.

The high-frequency induction heating was individually applied to the differential gear 1 one by one while carrying (conveying) it one by one. The water cooling in the rapid cooling period “d2” was applied for approximately 13 seconds, and the cooling rate during that period was 50° C./s to 65° C./s. During the water cooling, the differential gear 1 was rotated, and the cooling water was sprayed from around to the differential gear 1 to cool it one by one. In this way, the quenching step was performed with the method that can suppress occurrence of the distortion most effectively. The heat pattern in FIG. 18 is also an example, and can be changed, by conducting appropriate preliminary tests, to a heat pattern giving optimal conditions for the type of material steel used. For example, the cooling after the temperature increase can be performed in two stages.

The following describes the partial tempering step (the step 4) performed in the induction tempering machine 54. At the partial tempering step for the differential gear 1 and the test piece 20 used in the tests described above, the high-frequency induction heating was used as a means of high-density energy heating, and the partial heating shown in FIG. 10 was performed. The heat pattern shown in FIG. 19 was employed as the heat pattern at the edge part 16. In FIG. 19, as in FIGS. 17 and 18, the horizontal axis represents time, and the vertical axis represents temperature. In FIG. 19, the symbol “e1” represents the temperature increasing period, and the symbol “e2” represents a cooling period.

In the temperature increasing period “e1”, the amount of input energy was set to approximately 11 kW, and the heating time was set to approximately 5 seconds.

In this way, the heating temperature at the edge part 16 was increased to the range of 180° C. to 500° C. The surface layer on the upper end surface 14 (smaller diameter side) opposite to the edge part 16 was not heated, and thus remained at a room temperature of 20° C. to 25° C. The cooling in the cooling period “e2” was performed by water cooling. The cooling rate in this cooling period was set in the range of 80° C./s to 90° C./s. The cooling was performed at this cooling rate until the temperature decreases from the temperature at the end of the temperature increasing period “e1” to approximately 25° C.

As described above in detail, in the differential gear 1 of the present embodiment, in order to obtain the martensitic structure after the quenching, the C concentration after the carburizing is kept at a relatively low value by the vacuum carburizing. In this case, excess carbon enters the edge part 16, so the partial tempering step is performed after the quenching step is finished. In this way, while the content of C dissolved in the solid state in the martensitic structure in the old austenite grains of the edge part 16 is reduced, the portion other than the edge part 16 is prevented from being greatly affected by the reduction. The addition of, for example, Si secures the hardenability and the temper softening resistance. In this way, the differential gear 1 that has balanced strength at grain boundaries and in grains, and that has both sufficient hardness and sufficient fatigue strength as a drive system component for high-load use, and the method for manufacturing the differential gear 1 are achieved.

The present embodiment is merely an example, and is not intended to limit the present disclosure. Therefore, the present disclosure can naturally be improved and/or modified in various ways within the scope not deviating from the gist of the disclosure. 

1-8. (canceled)
 9. A gear that is formed of material steel molded to include a disc portion and a plurality of tooth portions circumferentially discretely formed on the disc portion and have a shape in which tooth root portions are formed between the tooth portions, and is subjected to vacuum carburizing treatment and subsequent quenching treatment with high-density energy heating after being molded, wherein the material steel has a chemical composition of: C: 0.10% by mass to 0.30% by mass; Si: 0.50% by mass to 3.00% by mass; Mn: 0.30% by mass to 3.00% by mass; P: 0.030% by mass or less; S: 0.030% by mass or less; Cu: 0.01% by mass to 1.00% by mass; Ni: 0.01% by mass to 3.00% by mass; Cr: 0.20% by mass to 1.00% by mass; Mo: 0.10% by mass or less; N: 0.05% by mass or less; and Fe and unavoidable impurities: a residual portion, where Si(% by mass)+Ni(% by mass)+Cu(% by mass)−Cr(% by mass)>0.5 is satisfied, a partially tempered region is provided in a surface layer of at least a part of a portion including an edge part at an end in an axial direction in the tooth portions and the tooth root portions, the partially tempered region has hardness lower than hardness of a martensitic structure generated in the surface layer of the part of the portion by the quenching treatment, and a surface layer of a portion other than the partially tempered region in the tooth portions and the tooth root portions is formed by the martensitic structure generated by the quenching treatment.
 10. The gear according to claim 9, wherein the chemical composition of the material steel further includes: B: 0.005% by mass or less; and Ti: 0.10% by mass or less.
 11. The gear according to claim 9, wherein a meshing region on each of tooth flanks meshing with another gear is not included in the partially tempered region, and is formed by the martensitic structure generated by the quenching treatment.
 12. The gear according to claim 10, wherein a meshing region on each of tooth flanks meshing with another gear is not included in the partially tempered region, and is formed by the martensitic structure generated by the quenching treatment.
 13. The gear according to claim 9, wherein the gear has a beveled shape in which an end thereof in the axial direction has a diameter larger than that of the other end, and the partially tempered region is provided to at least a part of the surface layer including the edge part at the end on the larger diameter side in the tooth portions and the tooth root portions.
 14. The gear according to claim 10, wherein the gear has a beveled shape in which an end thereof in the axial direction has a diameter larger than that of the other end, and the partially tempered region is provided to at least a part of the surface layer including the edge part at the end on the larger diameter side in the tooth portions and the tooth root portions.
 15. A side gear and a pinion gear in a differential device configured such that a plurality of gears according to claim 13 are meshed with each other, wherein a meshing region on each of tooth flanks of the gears meshing with the other of the meshing gears is not included in the partially tempered region, and is formed by the martensitic structure generated by the quenching treatment.
 16. A side gear and a pinion gear in a differential device configured such that a plurality of gears according to claim 14 are meshed with each other, wherein a meshing region on each of tooth flanks of the gears meshing with the other of the meshing gears is not included in the partially tempered region, and is formed by the martensitic structure generated by the quenching treatment.
 17. A method for manufacturing a gear that is formed of material steel molded to include a disc portion and a plurality of tooth portions circumferentially discretely formed on the disc portion and have a shape in which tooth root portions are formed between the tooth portions, wherein the material steel has a chemical composition of: C: 0.10% by mass to 0.30% by mass, Si: 0.50% by mass to 3.00% by mass, Mn: 0.30% by mass to 3.00% by mass, P: 0.030% by mass or less, S: 0.030% by mass or less, Cu: 0.01% by mass to 1.00% by mass, Ni: 0.01% by mass to 3.00% by mass, Cr: 0.20% by mass to 1.00% by mass, Mo: 0.10% by mass or less, N: 0.05% by mass or less, and Fe and unavoidable impurities: a residual portion, where Si(% by mass)+Ni(% by mass)+Cu(% by mass)−Cr(% by mass)>0.5 is satisfied is used, the method for manufacturing a gear comprising: a vacuum carburizing step of heating the gear formed of the molded material steel to a temperature at or higher than an austenitizing temperature of the material steel in a carburizing atmosphere at a pressure lower than the atmospheric pressure to form a carburized layer on a surface of the gear; a cooling step of cooling the gear, after the vacuum carburizing step, to a temperature lower than a temperature at which structure transformation due to the cooling is completed, at a cooling rate lower than a cooling rate at which the material steel is transformed into martensite; a quenching step of heating the gear after the cooing step with high-density energy heating to increase the temperature of the gear to a temperature at or higher than the austenitizing temperature of the material steel and, from that state, cooling the gear at a cooling rate at or higher than the cooling rate at which the material steel is transformed into martensite to form a martensitic structure at least in a portion of the carburized layer; and a partial tempering step of heating at least a part of a portion including an edge part at an end in an axial direction in the tooth portions and the tooth root portions of the gear with the high-density energy heating after the quenching step to increase the temperature of at least the part of the portion including the edge part to a temperature of 180° C. or higher at which austenitization of the material steel does not occur and, from that state, cooling the gear to reduce a concentration of carbon dissolved in a solid state in the martensitic structure in the portion of the carburized layer in at least the part of the portion including the edge part.
 18. The method for manufacturing a gear according to claim 17, wherein the chemical composition of the material steel used includes: B: 0.005% by mass or less; and Ti: 0.10% by mass or less.
 19. The method for manufacturing a gear according to claim 17, wherein the gear to be manufactured is a gear having a beveled shape in which an end thereof in the axial direction has a diameter larger than that of the other end and including the edge part at an end on the larger diameter side, and in the partial tempering step, the heating is performed in the state in which the end on the larger diameter side of the bevel-shaped gear lies in the space inside an exciting coil, and the end on the smaller diameter side of the bevel-shaped gear lies outside the exciting coil.
 20. The method for manufacturing a gear according to claim 18, wherein the gear to be manufactured is a gear having a beveled shape in which an end thereof in the axial direction has a diameter larger than that of the other end and including the edge part at an end on the larger diameter side, and in the partial tempering step, the heating is performed in the state in which the end on the larger diameter side of the bevel-shaped gear lies in the space inside an exciting coil, and the end on the smaller diameter side of the bevel-shaped gear lies outside the exciting coil. 